Dilute aqueous nitrite solutions are known to be effective anticorrosive agents for metals, particularly for iron-based metal parts. It is believed nitrites function by forming a protective oxide film on metal surfaces. Suitable concentrations required for anti-corrosion properties are about 500-1000 ppm of sodium nitrite (NaNO.sub.2). At higher concentrations of 2-4 percent by weight, the aqueous solutions also act as an aqueous solution freezing point depressant. However, it is also known that dilute aqueous nitrite solutions are unstable and gradually lose their ability to reduce corrosion. This occurs because of a loss of nitrite after exposure of the solution to the atmosphere. Prior art workers have disclosed that a bacteria known as Nitrobacter agilis, which is present in air, propagates in nitrite solution, via a process that oxidizes the nitrite ions to nitrate ions. Nitrate ions have no anticorrosion effects. Thus the nitrite ion becomes depleted and the efficacy of these solutions as anticorrosive agents is lost.
Lundgren et al, U.S. Pat. No. 3,087,777 disclose that a solution containing 500 ppm of nitrite ion initially, after exposure to the Nitrobacter agilis bacteria, lost all of its nitrite content in six days.
The bacterial action with nitrite solutions has been corroborated by Conoby et al, "Nitrite as a Corrosion Inhibitor", Materials Protection, April 1967 pp 55-58. The authors tested the use of dilute nitrite solutions for air conditioning systems. As long as the nitrite concentration, originally about 250-300 ppm as NaNO.sub.2, was maintained, no corrosion of the equipment was noted. However, rapid nitrite loss was noted in one tank, believed to be due to bacterial action, whereupon the anticorrosive effects of the nitrite solution disappeared.
Thus Lundgren et al suggested a variety of bactericides for this bacteria, which did extend the period of time the nitrite ion concentration remained stable in dilute solution. Further work with various bactericides has also been carried out by many prior art workers to overcome the bacterial action in dilute aqueous nitrite solutions. These bactericides include a variety of organic compounds and metal salts, such as chromates.
However these solutions find their way into ground water or sewage systems and, in accordance with present day environmental requirements, heavy metals and organic materials must first be removed from solutions prior to their disposal into sewage systems and waterways. This adds to the costs of using these solutions, and a ready means of disposal of the various pollutants is not always available. Thus it would be highly desirable if the use of bactericides for nitrite solutions could be avoided, while maintaining the nitrite content at a high enough level so that the anticorrosive properties of these solutions can be maintained.